CARACAS, Jan 9, 2013 (AFP) - Venezuela heads into uncharted political waters Wednesday without ailing President Hugo Chavez amid calls for the Supreme Court to decide if his government's postponing his inauguration is constitutional.

After days of suspense, the government confirmed Tuesday that Chavez, recovering in Cuba from cancer surgery, was still too sick to return for his re-inauguration Thursday and would take the oath of office at a later date before the Supreme Court.

Leaders of the leftist government insist that, under the circumstances, the president's current term can be extended beyond the January 10 inauguration date until he is well enough to be sworn in to another six-year term.

"If anyone has doubts, then go to the Supreme Court, go ahead to the Supreme Court, explain what your doubts are," Diosdado Cabello, the National Assembly speaker, said in a stormy debate after the delay was announced.

"We don't have any doubts about what we have to do and what is (stated) here in the constitution," he said.

The Supreme Court, which is controlled by pro-Chavez magistrates, called a news conference for Wednesday amid opposition demands for it to rule on the constitutionality of the government's decision.

On Tuesday, it rejected as inadmissible on technical grounds a challenge brought against Cabello's role, as the crisis deepened in this OPEC member which sits atop the world's largest proven oil reserves.

"I do not know what the judges of the Supreme Court are waiting for. Right now in Venezuela, without any doubt whatsoever, a constitutional conflict has arisen," opposition leader and former presidential candidate Henrique Capriles said.

Information Minister Ernesto Villegas said late Monday that Chavez's medical condition was unchanged since the latest complication from surgery was reported four days ago.

Chavez, who has not been seen in public for nearly a month, the longest stretch of his 14 years in power, is suffering from a severe pulmonary infection that has resulted in a "respiratory insufficiency," officials have said.

The announcement confirming that Chavez, 58, is too sick to be sworn in on the January 10 inauguration day came in a letter to the National Assembly from Vice President Nicolas Maduro.

"According to the recommendation of the medical team... the process of post-operative recovery must extend beyond January 10 of the current year, reason for which he he will not be able to appear on that date before the National Assembly," said the letter.

The letter went on to say that, in keeping with article 231 of the constitution, Chavez would take the oath before the Supreme Court at a later day.

In the National Assembly, deputies on both sides of the aisle stood up to make angry speeches for and against the government's decision to delay the swearing-in and extend his current term beyond January 10.

Taunting the opposition, Cabello said Chavez's followers would take to the streets to defend the government's decision.

"You can be certain that our people will be mobilized, defending the constitution, defending the law, defending the people, defending the decision that was taken," he said.

The government says the swearing-in is a mere formality that can be delayed, but the opposition says Chavez must at least be declared temporarily incapacitated and replaced on an interim basis by the National Assembly speaker.

Throughout his illness, first detected in June 2011, Chavez has refused to relinquish the powers of the presidency, even when leaving for Cuba for his fourth and most difficult round of surgery.

The charter says new elections must be held within 30 days if the president-elect or president dies or is permanently incapacitated either before he takes office or in the first four years of his six-year term.

Capriles also urged Latin American leaders -- Chavez has long been the figurehead of the anti-US left in the region -- to stay away from a rally convened by the government for Thursday in place of the inauguration.

So far Uruguay's President Jose Mujica, Bolivia's President Evo Morales and Foreign Minister Ricardo Patino of Ecuador have confirmed their attendance.

"Chavez's health is no longer in our hands," said Mujica in an interview with the Montevideo newspaper La Republica. "Our role is to back the government and people of Venezuela."

Capriles urged regional leaders not to succumb to "a game by a political party" -- alluding to Venezuela's ruling party.

Earlier, the country's main opposition coalition turned to international organizations for support, warning the Organization of American States of an "alteration of the constitutional order."